1. Field of the Invention
The present invention relates to an optical digital communication apparatus for transmitting and receiving digital data by using light.
2. Description of Related Art
As a conventional optical digital data communication apparatus is constituted by a transmission section and a reception section. In the transmission section, an infrared LED or a laser diode is driven by a digital electric signal corresponding to digital data to be transmitted, and an infrared signal corresponding to the data is radiated into space.
The reception section receives the infrared signal radiated from the transmission section by an infrared-receiving element such as a photodiode and convert the infrared signal into an analog electric signal. In addition, in the reception section, the obtained analog electric signal is amplified and then converted into a digital signal by a comparator or the like.
An infrared digital data communication apparatus generally performs communication between modules each having both of a transmission section and a reception section. However, the infrared digital data communication apparatus may perform communication between a module having only a transmission section and a module having only a reception section.
However, as one of factors which limits a communication distance, there is disturbance optical noise. As disturbance optical noise in the infrared digital data communication apparatus, noise such as sunlight, radiation light from an incandescent lamp or radiation light from a fluorescent lamp are known. In order to remove influence of such disturbance optical noise, various countermeasures are made in reception sections.
As a main countermeasure for removing influence of such disturbance optical noise, an arrangement of an optical filter, a device of the position of a light-receiving element, or the like is performed in an optical manner, and a method of using a filter is performed in an electric manner. However, according to these methods, shielding or filtering prevents receiving of disturbance optical noise, or received disturbance optical noise is removed by an electric filter. As a result, the disturbance noise cannot be completely removed.
For this reason, the present applicant has proposed an optical digital communication apparatus having the following arrangement in Japanese Patent Application No. 8-348878 (Japanese Patent Application Laid-Open No. 9-321705), Japanese Patent Application No. 8-348873 (Japanese Patent Application Laid-Open No. 9-321704), or the like. That is, two different optical signals are received to be converted into electric signals, and the level difference between the electric signals is detected to make it possible to remove a noise component.
An arrangement disclosed in Japanese Patent Application No. 8-348878 (Japanese Patent Application Laid-open No. 9-321705) is shown in FIG. 2 (to be referred to as a first prior art hereinafter). The optical digital communication apparatus shown in FIG. 2 converts a digital electric signal serving as an object of transmission into an optical signal in a transmission section 1, emits the optical signal from the transmission section 1 to a reception section 2, and converts the optical signal received by the reception signal 2 into an electric signal.
The transmission section 1 is constituted by a digital signal input terminal 11, a drive circuit 12, a light-emitting element 13, a linearly polarizing plate 14, and a xc2xc-wavelength plate 15.
The light-emitting element 13 is driven through the drive circuit 12 by a digital signal DS1 input to the digital signal input terminal 11 and serving as an object of transmission, and an optical signal is output from the light-emitting element 13.
In addition, the optical signal is linearly polarized by the linearly polarizing plate 14 arranged on the light-emitting side of the light-emitting element 13, and then is emitted by the xc2xc-wavelength plate 15 into space as a circularly polarized light component or an elliptically polarized light component.
On the other hand, the reception section 2 is constituted by xc2xc-wavelength plates 21A and 21B, linearly polarizing plates 22A and 22B, light-receiving elements 23A and 23B constituted by PIN photodiodes, and a subtraction circuit 24.
The light-receiving element 23A receives an optical light radiated from the transmission section and converts the optical signal into an electric signal DS2 to output the electric signal DS2. The light-receiving element 23B receives disturbance light scattered in space and converts the disturbance light into an electric signal DS2xe2x80x2 to output the electric signal DS2xe2x80x2.
A xc2xclength plate 21A and a linearly polarizing plate 22A are arranged on the light incident side of the light-receiving element 23A. The xc2xclength plate 21A and the linearly polarizing plate 22A convert circularly polarized light components or elliptically polarized light components radiated from the transmission section 1 into linearly polarized light components to cause the linearly polarized light components to be incident on the light-receiving element 23A.
The xc2xclength plate 21B and the linearly polarizing plate 22B are arranged on the light incident side of the light-receiving element 23B. The xc2xclength plate 21B and the linearly polarizing plate 22B convert circularly polarized light components or elliptically polarized light components having a rotating direction which is different from that of the circularly polarized light components or the elliptically polarized light component radiated from the transmission section 1 into linearly polarized light components to cause the linearly polarized light components to be incident on the light-receiving element 23B.
The output signals DS2 and DS2xe2x80x2 from the two light-receiving elements 23A and 23B are input to the subtraction circuit 24. An electric signal DS3 having a voltage level obtained such that the subtraction circuit 24 subtracts the voltage level of the electric signal DS2xe2x80x2 output from the other light-receiving element 23B from the voltage level of the electric signal DS2 output from the light-receiving element 23A is output.
On the other hand, the main electric system circuit of the transmission section 1 and the reception section 2 has the arrangement shown in FIG. 3. More specifically, the drive circuit 12 in the transmission section 1 is constituted by resistors 121 to 124 and a transistor 125, and the digital signal DS1 is input to one terminal of the resistor 121. The other terminal of the resistor 121 is connected to the base of the transistor 125 and one terminal of the resistor 122, and the emitter of the transistor 125 is connected to the one terminal of the resistor 123. A predetermined voltage +V is applied to the other terminals of the resistors 122 and 123. Furthermore, the collector of the transistor 125 is connected to the anode of the light-emitting element (LED) 13 through the resistor 124, and the cathode of the light-emitting element (LED) 13 is grounded.
With the arrangement, the transistor 125 is switching-operated in response to the digital signal DS1 to apply a voltage to the light-emitting element 13, thereby driving the light-emitting element 13.
In the reception section 2, the subtraction circuit 24 is constituted by a resistor 241 and an amplifier 242, and one terminal of the resistor 241 is connected to the anode of the light-receiving element (photodiode) 23A, the cathode of the light-emitting element (photodiode) 23B, and the input terminal of the amplifier 242, and the other terminal of the resistor 241 is grounded. Furthermore, a predetermined positive voltage +V1 is applied to the cathode of the light-receiving element 23A, and a predetermined negative voltage xe2x88x92V1 is applied to the anode of the light-receiving element 23B.
With the arrangement, a voltage having a difference between an output voltage of the light-receiving element 23A and an output voltage of the light-receiving element 23B is input to the amplifier 242.
With the arrangement described above, disturbance optical noise components equally input to the two light-receiving elements 23A and 23B are removed by the subtraction circuit 24. The electric signal DS2 output from one light-receiving element 23A has the same phase as that of the digital signal DS1 serving as an object of transmission, and the electric signal DS2xe2x80x2 output from the other light-receiving element 23B includes only a disturbance optical noise component. For this reason, when the difference between the voltage levels of these electric signals is calculated, the electric signal DS3 having the same phase as that of the digital signal DS1 serving as an object of transmission and an electric signal level output from the light-receiving element 23A can be obtained.
Therefore, disturbance optical noise can be removed, so that only a digital signal serving as an object of transmission can be obtained. In addition, influence of disturbance optical noise can be considerably reduced in comparison with the prior art, and a communication distance can be extended.
The optical digital communication apparatus disclosed in Japanese Patent Application No. 8-348873 (Japanese Patent Application Laid-Open No. 9-321704), as shown in FIG. 4 (to be referred to at the second prior art hereinafter), is designed such that two systems of optical signals can be emitted from the transmission section 1. In addition, a circularly polarized light component or an elliptically polarized light component which has the same phase as that of the digital signal DS1 serving as an object of transmission and can be received by one light-receiving element 23A is used as one optical signal, and a circularly polarized light component or an elliptically polarized light component which has a phase reversed to the phase of the digital signal DS1 and can be received by the other light-receiving element 23B is used as the other optical signal.
With the arrangement, disturbance optical noise components equally input to the two light-receiving elements 23A and 23B are removed by the subtraction circuit 24. An electric signal DS2 output from one light-receiving element 23A has the same phase as that of a digital signal DS1 serving as an object of transmission, and an electric signal DS2xe2x80x2 output from the other light-receiving element 23B has a phase which is different from that of the digital signal DS1 by 180xc2x0. For this reason, when the difference between the voltage levels of these electric signals is calculated, an electric signal DS3 having the same phase as that of the digital signal DS1 serving as an object of transmission and a level obtained by adding electric signal levels output from the two light-receiving elements 23A and 23B can be obtained.
Therefore, since the optical signals propagate as the circularly polarized light component or elliptically polarized light component in space, a variation in reception level caused by the rotating angles of the transmission section 1 and the reception section 2 is prevented, and disturbance optical noise is removed, so that only a digital signal serving as an object of transmission can be obtained. Influence of disturbance optical noise can be considerably reduced in comparison with the prior art, and a communication distance can be extended.
However, in the first and second prior arts described above, since the polarization angles of light components radiated from the light-emitting elements 13A and 13B (LED) of the transmission section 1 have isotropy, the intensities of linearly polarized light components emitted from the linearly polarizing plates 14A and 14B are half the intensities of light components radiated from the light-emitting elements 13A and 13B. For this reason, an S/N ratio decreases. In order to improve the S/N ratio, powers supplied to the light-emitting elements 13A and 13B must be increased. As a result, energy saving is hindered.
In addition, power supply circuits for generating two positive and negative voltages xe2x80x9c+V1xe2x80x9d and xe2x80x9cxe2x88x92V1xe2x80x9d in the reception section 2 are required, and simplification of the circuit and a decrease in size of the circuit are limited. When only a positive voltage xe2x80x9c+V1xe2x80x9d is used as one of the power supply circuits, the two light-receiving elements 23A and 23B are connected in series with each other. For this reason, the voltages applied to the light-receiving elements 23A and 23B are xc2xdxe2x80x9c+V1xe2x80x9d, and degradation of sensitivity caused by a decrease in signal amplitude, a decrease in response speed, and the like occur.
The present invention has been made in consideration of the above problems, and has as its object to provide a digital communication apparatus in which influence of disturbance optical noise is reduced, an S/N ratio is improved, and a communication distance can be extended.
The present invention, in order to achieve the above object, constitutes an optical digital communication apparatus comprising: a transmission section including a light-emitting element for emitting light in response to a digital signal serving as an object of transmission and optical signal emission means for dividing the optical signal emitted from the light-emitting element in two as two linearly polarized light components which are orthogonal to each other and for emitting the two linearly polarized light components as circularly polarized light components or elliptically polarized light components having the same rotating direction; and a reception section including first light-receiving means for receiving only circularly polarized light components or elliptically polarized light components having the rotating direction of the circularly polarized light components or the elliptically polarized light components emitted from the optical signal emission means of the transmission section to convert the circularly polarized light components or the elliptically polarized light components into an electric signal, second light-receiving means for receiving only circularly polarized light components or elliptically polarized light components having a rotating direction different from that of the circularly polarized light components or the elliptically polarized light components emitted from the optical signal emission means of the transmission section to convert the circularly polarized light components or the elliptically polarized light components into an electric signal, and subtraction means for outputting a difference between an electric signal output from the first light-receiving means and an electric signal output from the second light-receiving means.
In the optical digital communication apparatus with the arrangement described above, light having an intensity which is almost equal to that of light emitted from the light-emitting element of the transmission section. If attenuation in the air is not considered, almost all of optical signals radiated from the light-emitting element within a solid angle obtained when the light-receiving surface of the reception section is viewed from the light-emitting surface (position of wavelength plate) of the transmission section are received by the reception section, so that the optical signals can be reproduced as electric signals. More specifically, when the radiation intensity of light emitted from the light-emitting element is equal to that of a prior art, the intensity of light received by the light-receiving surface is twice that in the prior art. For this reason, an S/N ratio can be improved in comparison with the prior art, and energy saving can be achieved.
More specifically, since light emitted from the light-emitting element of the transmission section is divided in two as two linearly polarized light components which are orthogonal to each other, the intensities of the linearly polarized light components are xc2xd the intensity of the light emitted from the light-emitting element. One of the linearly polarized light components is emitted as a circularly polarized light component or an elliptically polarized light component having one rotating direction, and the other linearly polarized light component is emitted as a circularly polarized light component or an elliptically polarized light component having the same rotating direction as described above. Therefore, the sum of the light intensities of the two circularly polarized light components or the two elliptically polarized light components emitted from the transmission section is almost equal to the intensity of the light emitted from the light-emitting element.
In the reception section, both of the two circularly polarized light components or the two elliptically light components emitted from the transmission section and having the same rotating direction are received by the first light-receiving means. Therefore, the first light-receiving means receives almost all of optical signals radiated from the light-emitting element within a solid angle obtained when the light-receiving surface of the reception section is viewed from the light-emitting surface (position of wavelength plate) of the transmission section and scattered light in space. In addition, when the radiation intensity of light radiated from the light-emitting element is equal to that in the prior art, the intensity of the light received by the light-receiving surface is twice the intensity of light in the prior art. For this reason, in the electric signal output from the first light-receiving means, the intensity of a digital signal component serving as an object of transmission is twice the intensity of a digital signal component in the prior art.
The second light-receiving means receives a circularly polarized light or an elliptically polarized light component having the other rotating direction different from the rotating direction of the optical signal emitted from the transmission section. The circularly polarized light component or the elliptically polarized light component is scattered light in space.
In addition, the subtraction means outputs the difference between an electric signal output from the first light-receiving means and an electric signal output from the second light-receiving means. A scattered light component is removed by the subtraction. Therefore, an electric signal output from the subtraction means is a digital signal serving as an object of transmission.
The present invention constitutes an optical digital communication apparatus comprising: a transmission section including a first light-emitting element for emitting light in response to a digital signal serving as an object of transmission, first optical signal emission means for dividing an optical signal emitted from the first light-emitting element in two as two linearly polarized light components which are orthogonal to each other and for emitting the two linearly polarized light components as circularly polarized light components or elliptically polarized light components having a first rotating direction, a second light-emitting element for emitting light in response to a digital signal obtained by inverting the digital signal serving as an object of transmission, and second optical signal emission means for dividing an optical signal emitted from the second light-emitting element in two as two linearly polarized light components which are orthogonal to each other and for emitting the two linearly polarized light components as circularly polarized light components or elliptically polarized light components having a second rotating direction which is different from the first rotating direction; and a reception section including first light-receiving means for receiving only circularly polarized light components or elliptically polarized light components having the rotating direction of the circularly polarized light components or the elliptically polarized light components emitted from the first optical signal emission means of the transmission section to convert the circularly polarized light components or the elliptically polarized light components into an electric signal, second light-receiving means for receiving circularly polarized light components or elliptically polarized light components having the rotating direction of the circularly polarized light components or the elliptically polarized light components emitted from the second optical signal emission means of the transmission section to convert the circularly polarized light components or the elliptically polarized light components into an electric signal, and substraction means for outputting a difference between an electric signal output from the first light-receiving means and an electric signal output from the second light-receiving means.
The transmission section of the optical digital communication apparatus with the arrangement described above comprises the two arrangements of the transmission sections in the optical digital communication apparatus described above, and emits circularly polarized light components or elliptically polarized light components having different rotating directions. In addition, the circularly polarized light component or the elliptically polarized light component having one rotating direction is an optical signal corresponding to a digital signal serving as an object of transmission, and the circularly polarized light component or the elliptically polarized light component having the other rotating direction is an optical signal corresponding to a digital signal obtained by inverting the digital signal.
Therefore, light having an intensity which is almost equal to the intensity of light emitted from the first and second light-emitting elements of the transmission section is emitted as an optical signal. If attenuation in the air is not considered, almost all of optical signals radiated from the light-emitting element within a solid angle obtained when the light-receiving surface of the reception section is viewed from the light-emitting surface (position of wavelength plate) of the transmission section are received by the reception section, so that the optical signals can be reproduced as electric signals. More specifically, when the radiation intensity of light emitted from the light-emitting element is equal to that of a prior art, the intensity of light received by the light-receiving surface is twice that in the prior art. For this reason, an S/N ratio can be improved in comparison with the prior art, and energy saving can be achieved.
More specifically, since light emitted from the first light-emitting element of the transmission section is divided in two as two linearly polarized light components which are orthogonal to each other, the intensities of the linearly polarized light components are xc2xd the intensity of the light emitted from the first light-emitting element. One of the linearly polarized light components is emitted as a circularly polarized light component or an elliptically polarized light component having one rotating direction, and the other linearly polarized light component is emitted as a circularly polarized light component or an elliptically polarized light component having the same rotating direction as described above.
Therefore, the sum of the light intensities of the two circularly polarized light components or the two elliptically polarized light components having one rotating direction and corresponding to the optical signal emitted from the first light-emitting element of the transmission section is almost equal to the intensity of the light emitted from the first light-emitting element. The sum of the light intensities of the two circularly polarized light components or the two elliptically polarized light components corresponding to the optical signal emitted from the second light-emitting element of the transmission section is almost equal to the intensity of the light emitted from the second light-emitting element.
In the reception section, both the two circularly polarized light components or both the two elliptically polarized light components emitted from the transmission section and having one rotating direction are received by the first light-receiving means. In addition, both the two circularly polarized light components or both the two elliptically polarized light components emitted from the transmission section and having the other rotating direction are received by the second light-receiving means.
Therefore, the first and second light-receiving means receive almost all of optical signals radiated from the light-emitting element within a solid angle obtained when the light-receiving surface of the reception section is viewed from the light-emitting surface (position of wavelength plate) of the transmission section and scattered light in space. In addition, when the radiation intensity of light radiated from the light-emitting element is equal to that in the prior art, the intensity of the light received by the light-receiving surface is twice the intensity of light in the prior art. For this reason, in the electric signal output from the first and second light-receiving means, the intensity of a digital signal component serving as an object of transmission is twice the intensity of a digital signal component in the second prior art.
The circularly polarized light components or the elliptically polarized light components received by the first and second light-receiving means include digital signal components and scattered light components in space.
In addition, a difference between an electric signal output from the first light-receiving means and an electric signal output from the second light-receiving means is output by the subtraction means. With this subtraction, a scattered light component is removed. The digital signal component output from the first light-receiving means and the digital signal component output from the second light-receiving means are inverted to each other.
Therefore, the electric signal output from the subtraction means is a digital signal serving as an object of transmission.
In the present invention, the optical signal emission means described above is constituted by: a beam splitter, on which light emitted from the light-emitting element is incident, for emitting the incident light as two linearly polarized light components which are orthogonal to each other; a first wavelength plate, on which one linearly polarized light component emitted from the beam splitter is incident, for emitting the light component as a circularly polarized light component or an elliptically polarized light component having one rotating direction in a predetermined transmission direction; a second wavelength plate, on which the other linearly polarized light component emitted from the beam splitter, for emitting the light component as a circularly polarized light component or an elliptically polarized light component having one rotating direction; and guide means for causing the other linearly polarized light component emitted from the beam splitter to be incident on the second wavelength plate.
In the arrangement described above, light emitted from the light-emitting element is emitted by the beam splitter as two linearly polarized light components which are orthogonal to each other and have a light intensity which is almost xc2xd the intensity of the incident light.
One linearly polarized light component emitted from the beam splitter is converted by the first wavelength plate of xcex/4, xcex/5, or the like into a circularly polarized light component or an elliptically polarized light component having a first rotating direction, and the circularly polarized light component or the elliptically polarized light component is radiated into the air in a predetermined transmission direction. In addition, the other linearly polarized light component emitted from the beam splitter is incident on the second wavelength plate which is similar to the first wavelength plate by the guide means. The linearly polarized light component is converted by the second wavelength plate into a circularly polarized light component or an elliptically polarized light component having the first rotating direction, and the circularly polarized light component or the elliptically polarized light component is radiated into the air.
In the present invention, the reception section is constituted by: a third wavelength plate, on which a circularly polarized light component or an elliptically polarized light component having a first rotating direction is incident, for emitting a light component as a linearly polarized light component; a first linearly polarizing plate arranged to transmit only the linearly polarized light component emitted from the third wavelength plate; a fourth wavelength plate, on which a circularly polarized light component or an elliptically polarized light component having a second rotating direction different from the first rotating direction is incident, for emitting a light component as a linearly polarized light component; a second linearly polarizing plate arranged to transmit only the linearly polarized light component emitted from the fourth wavelength plate; a first light-receiving element, on which the linearly polarized light component emitted from the first linearly polarizing plate is incident, for converting the linearly polarized light into an electric signal; a second light-receiving element, on which the linearly polarized light component emitted from the second linearly polarizing plate is incident, for converting the linearly polarized light component into an electric signal; and a subtraction circuit for receiving the electric signals output from the first and second light-receiving elements and outputting a difference between these electric signal levels.
In the present invention, a photodiode having a cathode connected to a power supply is used as the first light-receiving element, and a photodiode having a grounded anode is used as the second light-receiving element. In addition, the subtraction circuit is constituted by: a first impedance element connected between the anode of the first light-receiving element and the ground and having a resistance lower than the resistance of the first light-receiving element; a second impedance element connected between the cathode of the second light-receiving element and a power supply and having a resistance lower than the resistance of the second light-receiving element; a first capacitance element connected between the anode of the first light-receiving element and a first output terminal; a second capacitance element connected between the cathode of the second light-receiving element and a second output terminal; and an arithmetic operation circuit for outputting an electric signal having a difference between an electric signal output from the first output terminal and an electric signal output from the second output terminal.
In this arrangement, a voltage applied across the anode and the cathode of a first photodiode is a voltage obtained such that a power supply voltage is divided by the resistance of the first photodiode and the resistance of the first impedance element when no light is incident on the first photodiode, and the potential of the anode of the first photodiode is lower than xc2xd of the power supply voltage. When the resistance of the first impedance element is set to be low, the potential of the anode can be set to be a ground potential, i.e., a potential which is approximate to 0 V.
When light is incident on the first photodiode, the resistance of the first photodiode decreases depending on the intensity of the incident light, and a voltage applied across the anode and the cathode of the first photodiode decreases. Accordingly, a voltage applied across both the terminals of the first impedance element decreases. Therefore, the potential of the anode of the first photodiode positively changes in response to the intensity of the incident light on the first photodiode.
On the other hand, a voltage applied across the anode and the cathode of a second photodiode is a voltage obtained such that a power supply voltage is divided by the resistance of the second photodiode and the resistance of the second impedance element when no light is incident on the second photodiode, and the potential of the cathode of the second photodiode is higher than xc2xd of the power supply voltage. When the resistance of the second impedance element is set to be low, the potential of the cathode can be set to be a potential which is approximate to the power supply voltage.
When light is incident on the second photodiode, the resistance of the second photodiode decreases depending on the intensity of the incident light, and a voltage applied across the anode and the cathode of the second photodiode decreases. Accordingly, a voltage applied across both the terminals of the second impedance element increases. Therefore, the potential of the cathode of the second photodiode negatively changes in response to the intensity of the incident light on the second photodiode.
In addition, an AC component at the anode of the first photodiode passes through a first capacitor, and an AC component at the cathode of the second photodiode passes through a second capacitor, so that these AC components are synthesized with each other.